The present invention relates to a device for improving performance and comfort of a motor vehicle.
In addition to the improvement of the passive safety of motor vehicles facilitated by furnishing the vehicles with crumple zones, safety belts and airbags to minimize passengers' risks in case of an accident, the improvement of the active safety of a vehicle is of the same or even higher importance. Active safety devices have the purpose to prevent accidents by controlling the behavior of the vehicle in situations which cannot be consistently mastered by every driver.
For this purpose, several systems have been developed, e.g. antilock brake systems (ABS), traction control systems (TCS) and yaw control systems (YCS) for supporting the driver during cornering. These systems have the goal to prevent unstable driving conditions that occur when a wheel begins to slip on the ground because the maximum static friction has been exceeded.
ABS, for instance, individually lowers the brake pressure during a braking operation actuated by the driver when a wheel threatens to lock. Thus, the respective wheel returns to the range of static friction, and driving stability is recovered.
Similarly, TCS reduces the driving torque when a driven wheel starts spinning. This can be carried out by applying a brake force to the respective wheel or by reducing the power applied to the driven wheels by the driving unit or both.
The YCS is even more sophisticated since it considers also the angular yaw velocity of the vehicle and side forces occurring to each single wheel.
The vector sum of all forces transferred between a wheel and the road surface cannot exceed a certain amount defined by the so-called circle of friction, the radius of which is proportional to the coefficient of friction between wheel and road surface.
In attempting to avoid excessive horizontal forces, it is of interest to know the coefficient of friction. Up to now, no consistent and satisfactory results have been achieved by efforts to measure the coefficient of friction. Therefore, the coefficient of friction usually is derived by processing data gained from wheel sensors detecting the individual wheel speeds.
Also an automation of comfort functions can contribute to the safety of a vehicle by avoiding a distraction of the driver when attention has to be paid to the traffic. Therefore, an automatic assistance of the driver, even comfort related, is desirable whenever outside conditions change.